Genetic analysis of galling responses in Vitis vinifera x Vitis rupestris hybrids to grape phylloxera (Daktulosphaira vitifoliae)

Grape phylloxera (Daktulosphaira vitifoliae Fitch), an insect pest of grapevines, is controlled by rootstocks bred from resistant North American grape species. However some rootstocks, such as the Vitis rupestris x V. vinifera hybrid AXR#1, have failed because the V. vinifera portion of their parentage is susceptible. Susceptible grapes form galls. Tuberosities are galls that form on larger lignified roots and lead to above-ground symptoms of water stress, reduced growth and fruit production, and eventual vine death. Nodosities occur on non-lignified roots and generally do not result in serious damage. These studies are the first to examine tuberosity and nodosity formation separately and place these traits on a genetic linkage map. F2 progeny from a remake of AXR#1 were screened with phylloxera originally collected on AXR#1 roots from California. The progeny were phenotypically classified as resistant or susceptible based on numbers of nodosities and tuberosities. Chi-square analyses on the phenotypes showed that segregation of nodosity formation fit a 1:7 phenotypic distribution, implying that at least two genes were involved. However, it was unclear whether one or two genes control tuberosity formation. We developed genetic linkage maps from the F2 progeny using simple sequence repeat (SSR) and amplified fragment length polymorphism (AFLP) markers. Quantitative trait locus (QTL) analyses detected one major QTL on linkage group 13 for nodosity formation. Two QTLs were discovered for tuberosity formation---one on group 9 and one on group 5. Markers closely linked to the QTLs will be useful in resistance breeding and serve as the basis for more detailed studies of the genetic mechanisms underlying the physical and chemical processes involved in galling. Grapevine susceptibility is also due to genetic differences in phylloxera strains, thus 96 populations in California were analyzed with 10 polymorphic SSR markers. Thirteen genotypes were found only in Monterey County and 39 genotypes were shared between individuals from Napa, Sonoma and Mendocino counties. A dendrogram showed most Monterey populations clustered together. These phylloxera were all collected from V. vinifera cultivars. The AXR# 1 populations from Napa, Mendocino, and Sonoma grouped into two additional clusters. This genetic similarity is based either on geographic location and/or root host.